An electroluminescent (EL) device is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. An organic EL device was first developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials to form a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
The organic EL device (OLED) converts electric energy into light when electricity is applied to an organic light-emitting material(s). Generally, the organic EL device has a structure comprising an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer of the organic EL device comprises a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer (comprising a host material and a dopant material), an electron buffering layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. Depending on its function, materials for forming the organic layer can be classified as a hole injection material, a hole transport material, an electron blocking material, a light-emitting material, an electron buffering material, a hole blocking material, an electron transport material, an electron injection material, etc. When a voltage is applied to the organic EL device, holes and electrons are injected from an anode and a cathode, respectively, to the light-emitting layer. Excitons having high energy are formed by recombinations between the holes and the electrons, the energy puts the organic light-emitting compound in an excited state, and the decay of the excited state results in a relaxation of the energy level into a ground state, accompanied by light-emission.
The most important factor determining luminous efficiency in the organic EL device is light-emitting materials. The light-emitting material needs to have high quantum efficiency, high electron mobility, and high hole mobility. Furthermore, the light-emitting layer formed by the light-emitting material needs to be uniform and stable. Depending on the colors visualized by light-emission, the light-emitting materials can be classified as a blue-, green-, or red-emitting material, and can additionally include a yellow- or orange-emitting material. Furthermore, the light-emitting material can be classified according to its function, as a host material and a dopant material. Recently, the development of an OLED providing high efficiency and a long lifespan is urgent. In particular, considering EL requirements for a middle or large-sized OLED panel, materials showing better performance than conventional ones must be urgently developed. In order to achieve the development, a host material which plays a role as a solvent in a solid state and transfers energy, should have high purity, and an appropriate molecular weight for being deposited under a vacuum. In addition, a host material should have high glass transition temperature and high thermal decomposition temperature to ensure thermal stability; high electrochemical stability to have a long lifespan; ease of preparation for amorphous thin film; and good adhesion to materials of adjacent layers. Furthermore, a host material should not move to an adjacent layer.
The light-emitting material can be prepared by combining a host with a dopant to improve color purity, luminous efficiency, and stability. Generally, a device showing good EL performances comprises a light-emitting layer prepared by combining a host with a dopant. The host material greatly influences the efficiency and lifespan of the EL device when using a host/dopant system, and thus its selection is important.
Korean Patent Application Laying-Open No. 10-2014-0096203 discloses an organic electroluminescent device using indole derivatives and biscarbazole compounds as a plurality of host materials. However, it does not specifically disclose an organic electroluminescent device comprising a compound having a structure wherein benzocarbazole and triazinyl are connected via a pyridine linker, as a host material.